(1) Field of the Invention
This invention relates to a method for disposal of sodium containing waste material. The method has utility in the pulp and paper industry.
(2) Description of the Prior Art
A waste product of the pulp and paper industry, known in the art as black liquor, is discharged from the pulp digestor during the paper making process and contains relatively high concentrations of sodium salts and bases such as, for example, sodium chloride, sodium hydroxide, sodium sulfates and sulfites and sodium carbonates.
A known method for the disposal of sodium containing waste material is to combust the material in various types of incinerators and then dispose of the combustion product. In this disposal method, metallic sodium is released during the combustion and reacts with the refractory materials that line interior portions of many conventional incinerators. The metallic sodium reacts with the refractory material to produce deterioration in the form of spalling, cracking and/or pitting. It is believed that the metallic sodium is attracted to the silica and/or the alumina which are standard composites in refractory material used in incinerators. The thus formed sodium silicates and sodium aluminates have a different density and a different coefficient of expansion than the base refractory material and, upon repeated heating and cooling, the sodium silicates and the sodium aluminates spall resulting in rapid deterioration of the refractory material and necessitating replacement.
One common method of extending the life of refractory material in an incinerator is to coat the surface of the refractory with a silica mortar which is higher in silica content than the base refractory. The silica in the silica mortar reacts preferentially with metallic sodium. This reaction creates a thick glassy phase external to the refractory material. The thick glassy phase serves as a partial barrier to sodium attack on the base refractory. Although this method extends the life of the refractory material, it is not effective in that the refractory material eventually spalls, at least partly because of thermal spalling resulting from cycling, and the refractory coating must be replaced. Another drawback of this method is that the requisite periodic shut-downs to reapply mortar are time consuming and thus expensive. Also, the high concentration silica mortar is expensive.
U.S. Pat. No. 3,537,815 discloses a method for the preparation of alkali metal silicates wherein a mixture of silica and a sodium containing compound is heated to a temperature between 3100.degree. F. and 3900.degree. F. to produce a sodium silicate. The patent discloses that furnace wall deterioration may be a problem. The patent states that the problem can be solved by combusting the mixture essentially out of contact with the furnace walls.
This solution to the problem of wall deterioration suggested in U.S. Pat. No. 3,537,815 is not practical in that it would require extensive redesign of incinerators in order to prevent contact of reactants with incinerator walls. Redesign and modification of installed incinerators would be very difficult. Also, this patent discloses that the reaction is carried out in a modified carbon black furnace but does not disclose that the furnace includes refractory materials which tend to deteriorate when exposed to metallic sodium.
U.S. Pat. No. 3,511,601 discloses a process for densifying rice hulls. This patent discloses that rice hulls, which contain about 20% silica, can be coated with sodium hydroxide and incinerated at a temperature of about 900.degree. C. to about 1500.degree. C. to produce sodium silicate which may be disposed of in a more simple manner than the product of incineration of rice hulls alone. This patent does not disclose the particular type of incinerator used in the process, nor does it disclose any type of problem incurred with deterioration of refractory materials by the reactants present.
Accordingly, it is an object of the present invention to provide a method for disposal of sodium containing waste material which minimizes deterioration of refractory materials used in incinerators and which does not require extensive redesign and modification of existing incinerators.
It is a further object of the present invention to provide a method for the simultaneous disposal of sodium containing waste material and rice hulls.
It is another object of the present invention to reduce the amount of fuel necessary to dispose of the reactants produced during incineration of sodium containing waste material by having one of the reactants provide some of the fuel.